Directional couplers are used in power control loops and in power amplifier linearization loops to sample the power of a radio frequency signal from a radio transmitter output. The sampled power may typically be 20 to 30 dB less than the output power of the transmitter. This coupled RF signal will be seen at a coupled output port of the directional coupler, and if the directional coupler is ideal, with infinite directivity, no power will be detected at an isolated port of the directional coupler.
An example of a known microstrip directional coupler 5 is shown in FIG. 1. The ideal directional coupler has the property that a wave incident in port 1 couples power to port 3 but not into port 4. Similarly, power incident in port 3 couples into port 1 but not into port 2. Thus, ports 2 and 4 are isolated. For waves incident in port 2 or 4, the power is coupled into the ports 4 or 2 only, so that ports 1 and 3 are isolated.
Directional couplers are widely used in impedance bridges for microwave measurements and for power monitoring. For example, if a radar transmitter is connected to port 1, an antenna to port 2, a microwave crystal detector to port 3, and a matched load to port 4, power received in port 3 is proportional to the power flowing from the transmitter to the antenna in the forward direction only. Since the reflected wave from the antenna, if it exists, is not coupled into port 3, the detector monitors the power output of the transmitter. In a practical directional coupler, some undesired power at the isolated port exists. This undesired power may appear as noise in power measurements and can reduce dynamic range and accuracy.
Microstrip directional couplers are ideally compact in size, use printed circuit board fabrication, are integrated with other circuitry on the printed circuit board, and provide a cost-effective solution compared to a strip line or waveguide directional coupler. The conventional microstrip directional coupler of FIG. 1 has a first microstrip 10 and a second microstrip 12 separated by a gap 13. The length of the coupled area of the microstrips are typically one quarter wavelength, and the directional coupler has quite flat coupling versus frequency characteristics, but a poor directivity. Directivity is defined as the ratio of desired power at the coupled port to the undesired power at the isolated port. The electromagnetic fields of the microstrip directional coupler exist in the dielectric and in the air. Because the even mode fields in the dielectric are slower than the odd mode fields in the air, the even and odd modes do not cancel in the reverse direction, making the directivity poor. Typically, directivity for quarter wavelength microstrip couplers is 7 to 15 dB, depending on frequency and coupling. High directivity is desired to prevent coupling of energy to an isolated port of the directional coupler. Also, the directional coupler is typically very large in physical size, especially for frequencies below 1 GHz.
One way to improve directivity is to make the directional coupler shorter than the usual quarter wavelength. For example, couplers that are an eighth of a wavelength long provide about 10 dB improvement in directivity. However, the coupling varies over the frequency significantly. To compensate for frequency variations, lumped circuit elements are used. For example, inductor 14 and impedance 16 connected to ground 18 are used to compensate for frequency variation of the isolated port 3. Impedance 19 connected to ground 18 represent a termination of the output port 4, such as a power detector. U.S. Pat. No. 5,129,298 discloses a microstrip directional coupler which uses a single compensating element, such as a capacitor or inductor, connected between the primary and secondary transmission paths of the coupler. U.S. Pat. No. 5,424,694, uses an inductor and parallel resistor in series with a coupled port. These solutions do not provide directivities above 20 dB. Further, compensating elements make the directional coupler design more complex and expensive, and take up more circuit board space.